SLCDs as defined in the preamble are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 096 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term SLCD here covers any more highly twisted display element with a value for the twist angle of between 160.degree. and 360.degree., such as, for example, the display elements of Waters et al. (C.M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T.J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
SLCDs of this type are distinguished, in comparison to standard TN displays, by significantly better steepnesses of the electrooptical characteristic line and consequently better contrast values, and by significantly less angle dependence of the contrast. Of particular interest are SLCDs having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures were hitherto optimized using usually monotropic additives having relatively high vapour pressure. However, the response times achieved were not adequate for all applications.
In order to achieve a steep electrooptical characteristic line in SLCDs, the liquid-crystal mixtures should have relatively large values for K.sub.33 /K.sub.11 and relatively small values for .DELTA..epsilon./.epsilon..sub..perp..
In addition to optimization of the contrast and the response times, further important requirements are made of mixtures of this type:
1. A broad d/p window PA1 2. Hich long-term chemical stability PA1 3. High electrical resistance PA1 4. Low frequency and temperature dependence of the threshold voltage. PA1 R.sup.b is alkyl or alkoxy having 1 to 5 carbon atoms, PA1 Z is --COO--, --CH.sub.2 CH.sub.2 -- or a single bond, and PA1 L.sup.a, L.sup.b, L.sup.c, L.sup.d, L.sup.e, L.sup.f, L.sup.g and L.sup.h in each case independently of one another are H or F, PA1 with the proviso that the 1,4-phenylene rings carry not more than 2 fluorine atoms per ring, PA1 and at least one compound of the formula IC ##STR4## in which PA1 they have low viscosity, PA1 they have low temperature dependence of the threshold voltage and the operating voltage, and PA1 they effect long storage times of the display at low temperatures.
The parameter combinations achieved are still far from. adequate, in particular for high-multiplex, but also for low- and medium-multiplex STNs (1/400). This is in some cases attributable to the fact that the various requirements are affected in opposite manners by material parameters.
There thus continues to be a great demand for SLCDs, in particular for high-resolution displays (XGA), having very short response times and at the same time a large operating temperature range, high characteristic line steepness, good angle dependence of the contrast and low threshold voltage which meet the abovementioned requirements.